Novel Mutations in TACSTD2 Gene in Families with Gelatinous Drop-like Corneal Dystrophy (GDLD)

In the current study, we conducted a mutation screening of tumor-associated calcium signal transducer 2 (TACSTD2) gene in six consanguineous Iranian families with gelatinous drop-like corneal dystrophy (GDLD), in order to find the causative mutations. Detailed eye examination was performed by ophthalmologist to confirm GDLD in patients. To detect the possible mutations, direct Sanger sequencing was performed for the only exon of TACSTD2 gene, and its boundary regions in all patients. In the patients with GDLD, the corneal surface showed lesions with different shapes from mild to severe forms depending on the progress of the disease. The patients showed grayish corneal deposits as a typical mulberry form, corneal dystrophy along with corneal lipid deposition, and vascularization. Targeted Sanger sequencing in TACSTD2 gene revealed the causative mutations in this gene in all studied families. Our study expanded the mutational spectrum of TACSTD2 which along with the related symptoms could help with the diagnosis, and management of the disease.

trait. GDLD appears in the first or second decade of life, and usually leads to blindness as its natural course (2). It is well-known that the causative gene for GDLD is tumor-associated calcium signal transducer 2 (TACSTD2), which was identified by Tdujikawa et al. in 1998 (3). TACSTD 2 has one exon with 972 nucleotides, and encodes for a protein with 323 amino acids (3). TACSTD 2 acts as a calcium signal transducer, and has receptor function activity (4). In patients with GDLD, superficial cells in cornea do not express claudin 1, tight junction protein 1, and occludin, in addition to stability change in cell-to-cell junctions. These findings suggest that any pathogenic alterations in TACSTD2 gene, due to loss of function, changes the permeability in epithelial cells that lead to clinical symptoms in cornea (5). Decreased epithelial barrier function, eases lactoferrin -an iron-binding glycoprotein present in exocrine fluids-deposition on the cornea, which eventually leads to amyloid deposition (6)(7). It has been reported that the phenotypic variability in GDLD could be due to age-related progression over time (8). However, a recent study in a consanguineous Colombian family with 5 affected patients suggested that environmental or other genetic factors may also affect the manifestation of clinical symptoms (8).
The other aspect of TACSTD2 gene, is its over expression relation with tumor cells. The role of TACSTD2 gene in progression and aggressiveness of colorectal cancer has been confirmed in different studies (9)(10)(11)(12), but coincidence of colorectal cancer in patients with GDLD has been less considered.
Prevalence of GDLD has not been determined in Iran, and previous studies have revealed some pathogenic mutations in Iranian population. In the current study, we investigated six consanguineous Iranian families with GDLD to detect the causative mutations and explain the symptoms related to each mutation and family. Six consanguineous Iranian families with at least two patients with GDLD were involved in our study. The pedigrees of these families are shown in Figure 1. The diagnosis of GDLD was performed by ophthalmologist, based on clinical manifestations, ocular examination, and histopathology evaluation of cornea tissue.

Ocular examinations
Uncorrected visual acuity was performed using the standard 20 feet Snellen acuity chart. Best The detected mutations were also confirmed using sequencing with the reverse primer.

Bioinformatics analysis
To assess the potential functional impact of
Some patients with amyloid deposition on the cornea have been reported previously that did not show pathogenic nucleoid alteration in TACSTD2 gene. In these cases, two other candidate genes transforming growth factor beta induced (TGFBI) and gelsolin (GSN) were assessed. Evidence showed that pathogenic nucleotide alteration in these two genes also lead to amyloid deposition, although their inheritance, and clinical signs are different from GDLD (14).
In conclusion, in the current study, we introduced three novel mutations in TACSTD2 gene which cause GDLD. Our study expanded the mutational spectrum of TACSTD2 gene, which along with the related symptoms could help with the diagnosis and management of the disease.
Functional studies are needed to clarify the molecular mechanisms which lead to GDLD due to mutations in TACSTD2 gene.